Instrument measuring chromatic dispersion in optical fibers

ABSTRACT

An instrument for measuring chromatic dispersion in optical fibers. The instrument comprises two independent modules, an optical source ( 2 ′) and a receiver ( 4 ′). The optical source ( 2 ′) comprises two pulse producing circuits: one driving a laser ( 35 ), which will be used as a timing reference, and the second driving a light emitting diode (LED)( 34 ), emitting a short light pulse with a broad spectrum. The receiver ( 4 ′) comprises a wavelength-selection element ( 46 ), which filters one or several wavelengths out of the LED spectrum, a photodetector ( 43 ) for detection of the laser pulse, a photon-counting detector ( 41 ) for detection of the filtered LED pulses, and a high-resolution timing circuit ( 42 ) for measuring the time delay between the laser pulse and the LED pulse. A preferred embodiment includes the wavelength-selection element ( 46 ) which is a series of Bragg gratings, thus selecting several fixed wavelengths.

This application claims the benefit of Provisional Application No.60/227,437, filed Aug. 22, 2000.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of opticaltelecommunications, and more particularly to an instrument for measuringchromatic dispersion in optical fibers.

In general, all optical waveguides, and optical fibers in particular,exhibit dispersion. Dispersion generates a temporal broadening ofoptical pulses, which are the information carriers in opticaltelecommunications. This ultimately limits the transmission rate of thechannel: the pulses overlap, and the ability to separate them is lost.In single-mode optical fibers the main factor of dispersion is chromaticdispersion: each frequency, or wavelength (i.e. color in the visibledomain), propagates at a slightly different velocity. Since an opticalpulse is built of a range of wavelengths, each of them propagating at adifferent velocity, chromatic dispersion is the main factor causingbroadening of the pulses. Measuring chromatic dispersion in opticalfibers, and compensating it with so-called chromatic dispersioncompensators has become an essential issue.

In its recommendation ITU-T G.650, the International TelecommunicationUnion proposes three possible techniques for measuring the chromaticdispersion coefficient, which characterizes chromatic dispersion: thephase shift technique; the interferometric technique; and the pulsedelay technique. For all these techniques, the chromatic dispersioncoefficient is derived from a measurement of the relative group delayexperienced by various wavelengths during propagation through a knownlength of fiber. The interferometric technique is designed formeasurements of short lengths of fibers (having a length of meters). Itis not adapted to long fibers (several kilometers) and installed cables,which is the domain addressed by the invention. Therefore, it will notbe discussed further.

In the phase shift technique, which has been chosen as the referencetechnique, the group delay is measured in the frequency domain, bydetecting, recording and processing the phase shift of a sinusoidalmodulating signal. This method and some variations are used in allavailable commercial instruments, and are covered by several patents(references: U.S. Pat. No. 5,033,846 by Hernday et al., U.S. Pat. No.5,406,368 by Horiuci et al.). This technique allows the most precisedetermination of the group delay (to the sub-picosecond), but requiresdelicate and expensive instruments.

The third method, based on a direct measurement of the group delay, hasnot yet found its way into commercial practice because it is difficultto combine in a satisfactory way the three main ingredients required:(1) fast optical pulses, either tunable or with a large spectrum; (2)fast detection, sensitive enough to detect the pulses; and (3) highresolution timing circuit.

Therefore, what is needed is a device and/or method of accuratelymeasuring the chromatic dispersion coefficient over long distances whichdoes not require delicate or expensive instruments.

SUMMARY OF THE INVENTION

An instrument is provided for measuring chromatic dispersion in opticalfibers. The instrument comprises two independent modules, an opticalsource and a receiver. The optical source comprises two pulse producingcircuits: one driving a laser, which will be used as a timing reference,and the second driving a light emitting diode (LED), emitting a shortlight pulse with a broad spectrum. The receiver comprises awavelength-selection element, which filters one or several wavelengthsout of the LED spectrum, a photodetector for detection of the laserpulse, a photon-counting detector for detection of the filtered LEDpulses, and a high-resolution timing circuit for measuring the timedelay between the laser pulse and the LED pulse.

The primary object of the invention is to provide a chromatic dispersionanalyzer for optical fibers with two independent modules: a source and areceiver. The source and the receiver are only connected via the fiberunder test, and need no extra communication between them.

Another object of the invention is to provide high-speed measurement ofchromatic dispersion.

Another object of the invention is to provide an instrument with highdynamic range.

A further object of the invention is to have a small lightweight source,with low power consumption, so as to allow a battery-powered option.

Yet another object of the invention is to allow a cost efficientmulti-source configuration, with interchangeable sources.

In accordance with a preferred embodiment of the present invention, aninstrument for measuring chromatic dispersion in optical fiberscomprises:

an optical source, including two pulse producing circuits: one driving alaser, which will be used as a timing reference; and the second drivinga light emitting diode (LED), emitting a short light pulse with a broadspectrum;

a receiver including a wavelength-selection element, which filters oneor several wavelengths out of the LED spectrum; a photodetector fordetection the laser pulse; a photon-counting detector (“PCD”) fordetection of the filtered LED pulses; and a high resolution timingcircuit for measuring the time delay between the laser pulse and the LEDpulse;

and a counting circuit, for recording the time delays for a large numberof pulses.

Other objects and advantages of the device will become apparent from thefollowing descriptions, taken in connection with the accompanyingdrawings, wherein, by way of illustration and example, an embodiment ofthe present invention is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments to the invention, which may be embodied in variousforms. It is to be understood that in some instances various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

FIG. 1 is a schematic diagram illustrating the operation of the opticalsource.

FIG. 2 is a schematic diagram illustrating the operation of thereceiver.

FIG. 3 is a schematic diagram illustrating the operation of the opticalsource of another embodiment of the instrument.

FIG. 4 is a schematic diagram illustrating the operation of the receiverof another embodiment of the instrument.

FIG. 5 is a schematic diagram illustrating the operation of the receiverof yet another embodiment of the instrument.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed descriptions of the preferred embodiment are provided herein.It is to be understood, however, that the present invention may beembodied in various forms. Therefore, specific details disclosed hereinare not to be interpreted as limiting, but rather as a basis for theclaims and as a representative basis for teaching one skilled in the artto employ the present invention in virtually any appropriately detailedsystem, structure or manner.

Referring now to FIGS. 1 and 2, a first embodiment of the invention isdescribed. In this embodiment, the instrument includes an emitter orsource 2, described in FIG. 1 and a receiver 4, described in FIG. 2.

An electronic trigger (“ET”) 11 triggers pulsers (“P”) 12 and 13 atregular intervals. When triggered by the electronic trigger 11, pulser12 generates a current pulse, driving laser 15. Concomitantly pulser 13generates a current pulse, driving the light emitting diode (LED) 14. Anoptical delay line 16 ensures that the laser pulse precedes the LEDpulse in the receiver. The laser pulse goes through coupler 17 directlytowards the output. The broadband LED pulse is first sent towards awavelength selection device 18. This device 18 is made of a series ofBragg gratings, each one reflecting light at a given wavelength, denotedby λ1 to λ5. Note that the number and the values of the wavelengths canbe chosen arbitrarily. This device 18 effectively slices out a number ofdifferent wavelengths out of the full LED spectrum, and reflects themtowards coupler 17 to the output.

Now referring to FIG. 2, the receiver 4 comprises a detector 21, whichis an avalanche photodiode (APD) used in two modes in succession: firstthe standard mode, which detects the laser pulse and then thephoton-counting mode, which detects the LED pulse. The photon-countingmode of the APD is enabled by a bias voltage on the diode, sent by thetiming circuit (“TC”) 22. This time window is triggered by the initiallaser pulse arriving on 22. Since the same detector is used for both thelaser pulse and the LED pulse, the time delay needed to compensate theinitial delay line 16 in the emitter cannot be obtained with an opticaldelay line, but has to be obtained with the electronic delay line(“EDL”) 23.

The above embodiment, however, has its shortcomings. The primaryshortcoming is the use of a single detector 21 for detection of both thelaser pulse and the LED pulse. This introduces a large noise in thephoton-counting detection, which limits the dynamic range of theinstrument. This noise is due to the so-called after pulses in the APD.During the detection of a strong pulse, a large number of chargedcarriers are generated in the APD. Some of these carriers are trapped bydefaults in the device and remain there for a while. When thephoton-counting mode, with a large bias voltage, is triggered, thesetrapped carriers are freed. This generates immediately a count; similarto the one obtained when a photon is detected. This effect can bereduced by using a long delay between the laser pulse and the LED.However, compensating this long delay with an electronic delayintroduces significant time jitter, which reduces the accuracy of themeasurement.

Therefore, it is important to ensure that no strong pulse impinges onthe photon-counting detector before the photon-counting mode is enabled.This is obtained in the subsequently described embodiment in which twodetectors 41 and 43 are used for detection of the laser pulse.

Now referring to FIGS. 3 and 4, in which a preferred embodiment isshown, an electronic trigger 31 triggers pulsers 32 and 33 at regularintervals, typically every ten microseconds. When triggered by trigger31, pulser 32 generates a current pulse, driving laser 35. Concomitantlypulser 33 generates a short current pulse, driving LED 34. Typically,the LED pulse lasts about 300 to 500 ps. Such short pulses are needed toobtain the high temporal precision required in chromatic dispersionmeasurements. An electronic delay line 36 is inserted before the laser35, to ensure that the LED pulse precedes the laser pulse in the deviceunder test and at the input of the receiver 4′ of FIG. 4. An opticalcoupler 37 mixes the two light pulses. The output of the optical sourceis thus a series of double optical pulses, first a short LED pulse, withlarge frequency spectrum, followed by a laser pulse.

Calibration of the instrument is performed by connecting the receiver 4′directly to the source 2′ of FIG. 3. The two pulses generated by thesource go through coupler 45. The LED pulse enters the wavelengthselection device 46. This device is made of a series of Bragg gratings,each one reflecting light at a given wavelength, denoted by λ1 to λ5.Note that the number and the values of the wavelengths can be chosenarbitrarily. This device effectively slices out a number of differentwavelengths out of the full LED spectrum, and reflects them towards thephoton-counting detector 41, with a given time interval between them,corresponding to the distance between the gratings. The laser pulse isdetected by detector 43, which sends the “start” signal to thehigh-resolution timing circuit 42. Upon reception of this “start”signal, the circuit sends a bias voltage towards the APD, thus enablingthe photon-counting mode. The optical delay line 44 is designed toensure that the photon-counting mode of detector 41 is enabled beforethe LED optical pulse reflected by 46 arrives in the detector 41. Inphoton-counting mode, the photon-counting detector 41 behavesessentially as a digital device, giving a single count at the arrivaltime of the photon. An optical attenuator 47 is set to ensure that nomore than one photon is present within the whole series ofwavelength-selected pulses. This photon can arrive at random, at any ofthe times corresponding to the wavelength-selected pulses. The timing ofthe count provides the “stop” signal for the high-resolution timingcircuit 42. The interval “start-stop” gives a delay, which is fed to thecounting device 48. Processing a large number of these generates ahistogram, with a precise time of detection for each of the selectedwavelengths. The time difference between each wavelength, when a source2′ is directly connected to the receiver 4′ gives the calibration of theinstrument. Note that this calibration needs to be done only once, anddoes not depend on the source. Therefore, one advantage of thisembodiment is that different sources can be used in conjunction with agiven receiver, with no need for recalibration of the system.

To perform a measurement, the fiber under test is inserted between thesource 2′ and the receiver 4′. During the propagation, chromaticdispersion of the fiber under test broadens the LED pulse: eachfrequency component of the LED takes a slightly different time. Thismodifies the relative delay between the wavelength-selected pulsesgenerated by the wavelength selection device 46. Comparing the delaysobtained with the fiber under test to the calibrated ones directlyyields the group delay for each of the selected wavelengths. Thechromatic dispersion coefficient can be inferred from the group delay,according to the ITU-T recommendation G.650.

The extreme sensitivity of the photon-counting device, together with theafter pulses already mentioned above, require that no light shouldimpinge on detector 41 before the signal from the LED, reflected by thewavelength selecting device 46, arrives on the detector 41. For thisreason, the strong laser pulse is delayed in the source, with respect tothe LED pulse. In addition, the receiver 4′ is designed to avoid anyreflection, even from the LED pulse, entering the detector 41 before thesignal. For example, some of the LED pulse is split by coupler 45 and isreflected at the end-face of the fiber, before detector 45. The distancebetween coupler 45 and detector 43 has to be longer than the distancebetween the detector 45 and the reflectors in the wavelength selectiondevice 46.

In some cases a very high dynamic range of the device is needed. This isthe case for very long optical links, of about 200 km. In this case, itis possible to increase the dynamic range by a modification of thereceiver 4″, as shown in FIG. 5. This modification requires an extraoptical component, making the system more expensive. It is therefore notnecessary for most systems.

Now referring to FIG. 5, in which the main difference with FIG. 4 isthat the coupler 45 has been replaced by an optical circulator 55, lightentering through port a goes to port b, while light entering throughport b goes to port c. This device therefore suppresses the lossintroduced by the coupler 45 of the previous embodiment. The two pulsesgenerated by the source enter port a of the circulator, and go to portb. The selected wavelengths of the LED pulse are reflected by thewavelength selecting device 56, while the laser pulse is detected indetector 53. In this case, the wavelength of the laser has to bedifferent from all the selected wavelength of the wavelength selectingdevice 56. The reflected wavelengths enter port b of circulator 55, andgo to port c, towards the optical delay line 54, the optical attenuator57 and the photon-counting detector 51. The timing circuit 52 and thecounting device 58 are identical to the high-resolution timing circuit42 and the counting device 48 respectively.

In an advantage, the invention provides a low cost, fast and reliablemeasurement of the chromatic dispersion.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

1. An instrument for measuring chromatic dispersion in a subject opticalfiber including an optical source and a receiver, wherein: the opticalsource for connection to an input end of the subject optical fiber,includes (a) a first pulse producing circuit which drives a laser thatproduces a laser pulse, the pulse entering the input end of the subjectfiber, said laser pulse being used as a timing reference; and (b) asecond pulse producing circuit which drives a light emitting diode (LED)which emits a short light pulse with a broad spectrum, which is alsocoupled to the subject optical fiber; the receiver including (a) aphotodetector, connected at the output end of the subject optical fiberfor detecting the laser pulse; (b) a wavelength-selection elementconnected to the output end of the subject optical fiber and thusoptically connected to the LED, which filters one or several wavelengthsout of the LED spectrum of the light pulse and transmits them tophoton-counting detector; (c) the photon-counting detector, connected atthe output end of the subject optical fiber, for detection of thefiltered LED pulses; and (d) a high resolution timing circuit formeasuring the time delay between the laser pulse and the LED pulse,connected to the photodetector at a receiving end of the timing circuitand, connected to the photo-counting detector at a feedback end of thehigh resolution timing circuit, and, connected to an analysis circuitand an output end of the high resolution timing circuit wherein theanalysis circuit is a counting circuit which records the time delays fora large number of pulses.
 2. The instrument of claim 1 wherein saidwavelength-selection element is a series of several Bragg gratingsinscribed in a fiber, thus selecting several fixed wavelengths.
 3. Theinstrument of claim 1 wherein said wavelength-selection element is atunable filter, whose central wavelength is sweeped during themeasurement.
 4. The instrument of claim 3 wherein said tunable filter isa monochromator.
 5. The instrument of claim 3 wherein said tunablefilter is a Fabry-Perot cavity.
 6. The instrument of claim 3 whereinsaid tunable filter is an acousto-optic element.
 7. The instrument ofclaim 1 wherein said photon-counting detector is an avalanche photodiode(APD), on which a bias voltage is applied during a well-defined timewindow, thus enabling the photon-counting mode during this window. 8.The instrument of claim 7 further comprising optical and electroniccomponents, such as delay lines, optical filters, optical isolators andoptical circulators; said components being chosen to ensure that saidphoton-counting window is triggered by the detected laser, whileensuring that no light impinges on said photon-counting detector beforethe signal from the wavelength-selection element.
 9. The instrument ofclaim 8 wherein said components are a delay line in the source,generating a constant time delay between the LED pulse and the laserpulse; and an optical delay line in the receiver; said delay lines beingchosen to ensure that the photon-counting window is triggered by thelaser pulse, while ensuring that all reflections from said opticalpulses, with the exception of the signal from the wavelength-selectionelement, arrive at the photon-counting detector after the end of thetime window.
 10. The instrument of claim 1 wherein said high-resolutiontiming circuit is a time-to-digital-converter.
 11. The instrument ofclaim 1 wherein said counting circuit is an external computer.
 12. Aninstrument for measuring chromatic dispersion in a subject optical fibertransmitting a laser timing pulse, the pulse entering the input end ofthe subject optical fiber along with a short light LED pulse with abroad spectrum, the instrument comprising a receiver including (a) aphotodetector, connected at the output end of the subject optical fiberfor detecting the laser pulse; (b) a wavelength-selection elementconnected to the output end of the subject optical fiber which filtersone or several wavelengths out of the LED spectrum of the light pulseand transmits them to a photon-counting detector; (c) thephoton-counting detector, connected at the output end of the subjectoptical fiber, for detection of the filtered light pulses; and (d) ahigh resolution timing circuit for measuring the time delay between thelaser pulse and the LED pulse, connected, to the photodetector at areceiving end of the high resolution timing circuit and connected to thephoton-counting detector at a feedback end of the high resolution timingcircuit, and connected to an analysis circuit at an output end of thehigh resolution timing circuit wherein the analysis circuit is acounting circuit which records the time delays for a large number ofpulses.
 13. The instrument of claim 12 wherein said wavelength-selectionelement is a series of several Bragg gratings inscribed in a fiber, thusselecting several fixed wavelengths.
 14. The instrument of claim 12wherein said wavelength-selection element is a tunable filter, whosecentral wavelength is sweeped during the measurement.
 15. The instrumentof claim 14 wherein said tunable filter is a monochromator.
 16. Theinstrument of claim 14 wherein said tunable filter is a Fabry-Perotcavity.
 17. The instrument of claim 14 wherein said tunable filter is anacousto-optic element.
 18. The instrument of claim 12 wherein saidphoton-counting detector is an avalanche photodiode (APD), on which abias voltage is applied during a well-defined time window, thus enablingthe photon-counting mode during this window.
 19. The instrument of claim18 further comprising optical and electronic components, such as delaylines, optical filters, optical isolators and optical circulators; saidcomponents being chosen to ensure that said photon-counting window istriggered by the detected laser, while ensuring that no light impingeson said photon-counting detector before the signal from thewavelength-selection element.
 20. The instrument of claim 12 whereinsaid high-resolution timing circuit is a time-to-digital-converter. 21.The instrument of claim 12 wherein said counting circuit is an externalcomputer.